1. Field of the Invention
The present invention relates to control circuitry for electrical devices and, more particularly, to control circuitry for electrical devices, such as contactors, starters and the like having one or more pairs of separable main contacts that are controlled by a dual winding coil having a starting winding and a serially connected holding winding, wherein the holding winding is shorted until the separable main contacts have closed, which includes circuitry which obviates the need for relatively precise sensing of the position of the separable main contacts to control the holding winding.
2. Description of the Prior Art
Various electrical devices are known in the art for controlling electrical equipment, such as motors and the like. These devices include starters, combination starters, contactors and the like for controlling both single phase and multiple phase electrical equipment in reversing as well as non-reversing applications. Such devices include one or more pairs of separable main contacts disposed between such electrical equipment and a source of electrical power. The separable main contacts are generally intended to interrupt the normal rated current of the electrical equipment and limited overcurrent conditions (e.g., up to approximately 10 x rated current) and are not intended to interrupt during a severe overcurrent condition, such as a short circuit condition. Overcurrent protection is generally provided by other devices, such as circuit breakers, fuses, and the like. Such overcurrent devices either can be included with the device or provided externally, depending on the particular application.
In such devices, the separable main contacts are mechanically interlocked with an armature that is controlled by a dual winding coil having a starting winding and a series connected holding winding. A biasing spring or gravity maintains the separable main contacts in a normally open position. The dual coil winding is normally under the control of an electrical interlock, such as an on and off switch, to allow the separable main contacts to be closed on command. Since the biasing force of the spring, which can exceed 100 pounds, must be overcome in order to close the separable main contacts, the holding winding is normally shorted out to allow relatively high electrical current to flow through the starting winding generating a relatively high force via the high ampere turns which causes the separable main contacts to close. However, once the separable main contacts are closed, the force required to maintain that position is significantly less than the force required to initially overcome the spring force. Therefore, the holding winding is unshorted thereby reducing the electrical current to the starting winding.
It is known in the art to use a mechanical interlock, such as a limit switch (often designated as L63) to sense the armature position to determine when the separable main contacts are in a closed position for purposes of shorting and unshorting the holding winding. The limit switch is normally provided with a normally closed contact, connected in parallel across the holding winding. Since the limit switch contact is subjected to the relatively large starting current as well as the inductive kick resulting from the sudden change of electrical current through the windings due to the unshorting of the holding winding, the limit switch must be provided with a relatively heavy duty contact which can be relatively expensive, thus increasing the cost of the device.
Additionally, in known devices the adjustment of the limit switch is fairly critical. More specifically, since the starting winding is only intended to conduct relatively high electric starting currents for a relatively short period of time, the starting winding normally is provided with an intermittent rating to handle the relatively large starting current. Consequently, it is necessary that the limit switch be adjusted relatively precisely to unshort the holding winding to reduce the electrical current through the starting winding relatively quickly after the separable main contacts are closed to avoid overheating. Moreover, improper adjustment of the limit switch can cause misoperation of the device. More specifically, if the holding winding is unshorted prematurely the electrical current through the windings will be significantly reduced (e.g., prior to the closure of the separable main contacts), this will result in chattering of the armature and failure to fully close the separable main contacts. This can result in device failure by either the main contacts welding or the coil burning out.